Github - Merging is blocked

I have multiple branches: master, development and stav. (There is only one developer in the repository at the moment - me).

master and development branches are fully protected (all the checkboxes are selected). I need to push to stav and create a pull request for merging to development.

stav branch have development branch chanages:

*   47e7341 - (21 minutes ago) Merge remote-tracking branch 'origin/development' into stav - stav alfi (HEAD -> stav, origin/stav)
|\
| *   afa89f6 - (30 minutes ago) Merge pull request #1 from UniversityFinalProjects/stav - stavalfi (origin/development)
| |\
* | | cd93730 - (25 minutes ago) 8 - stav alfi
| |/
|/|
* | 9336798 - (51 minutes ago) 8 - stav alfi
* | c2c240e - (60 minutes ago) 7 - stav alfi
* | 8104fdb - (81 minutes ago) 6 - stav alfi
* | d48f373 - (83 minutes ago) 5 - stav alfi
* | 7bac29b - (3 hours ago) 4 - stav alfi (master)
|/
* cfc2572 - (6 hours ago) 1 - stav alfi (origin/master, topic, development)
* 686e526 - (6 hours ago) Initial commit - stav alfi

Github doesn't let me finish the pull request:

The error:

Merging is blocked. Merging can be performed automatically with one approved review.

What does it mean? How can I fix it?

That comes from late 2015 "Protected branches and required status checks" where protected branch can have required status checks.

See "About required status checks" and "About required reviews for pull requests"

If a person with admin permissions chooses the Request changes option in a review, then that person must approve the pull request before it can be merged

Note: a review can be dismissed (although only by the owner/admin of the repo).

As torek comments, once a review is performed, the "Merging is blocked" cleared out automatically.